1. Field of the Invention
The present invention relates to novel wholly aromatic polyamides and copolyamides with new structures, process for preparing the same polyamides and films resulted from the same polyamides.
2. Description of the Prior Art
It has been known that since in the conventional high performance aromatic polyamides the constituent molecular chains are very stiff, those materials manufactured from the aromatic polyamides of such type, for example, films, fibers, etc., tend to undergo fibrillation resulting in cleavage and are difficult to stretch. Owing to these drawbacks, the durability becomes lowered rapidly when the materials are used in the place to which an external force is applied in traverse direction or a repeated load is applied.
On the other hand, aliphatic polyamides of flexible molecular chains have high elongation and low tendency to fibrillation. However, these aliphatic polyamides fail to meet the requirements of the high performance heat resistant materials due to poor strength, low modulus and shortage of heat resistance thereof.
Aromatic polyamides not only have a high glass transition temperature and a high melting point (in practice, there are many cases where decomposition occurs at a temperature below the melting point), but also are superior in terms of thermal stability and chemical resistance. Thus, the materials resulted from such aromatic polyamides are suitable for use in the utility where high heat resistance and weather resistance are required. Despite these advantages, the aromatic polyamides suffer from defects that they are still difficult to mold into desired materials.
Fibers having ultrahigh strength and modulus can be prepared by using the aromatic polyamides or copolyamides of the molecular chains straightly extended from both ends of the aromatic ring toward its parallel direction. For example, such fibers include the "Kevlar" fiber, commercially available from E. I. Du Pont de Nemours and Company, Wilmington, Del. U.S.A., which is prepared from poly(p-phenyleneterephthalamide. In spite of their outstanding stiffness and symmetry, the aromatic polyamides encounter with the difficulty in molding into desired materials due to their tendency to be decomposed below the melting point as well as the significantly low solubility in organic solvents. Practically, poly(p-phenyleneterephthalamide) is dissolved in very limited kinds of solvents. The representative solvents may include strong inorganic acids, e.g., conc. sulfuric acid, and a mixture of an alkali metal salt, e.g., LiCl in hexamethylphosphoamide and N-methylpyrrolidone. Among these, the solvent which is useful for the purpose of molding polymer is strong inorganic acids only.
The use of strong inorganic acids may cause various problems such as corrosion of the installation and equipment, dangers and difficulties in handling, treatment of waste liquid and intricate of operations for dissolving the polymers in the solvent. In addition, when using sulfuric acid as the solvent, the sulfuric acid rapidly leave from the polymer molecular chains when the coagulation occurs in the polymerization solution after molding. The sulfuric acid thus left accelerates the fibrillation which is a drawback of linear stiff chains. The fibrillation of the molded article accelerated by the inorganic acid may be one of the fatal drawbacks in the aramid materials having various utility as the reinforcing materials. In spite of these disadvantages, fibers having high strength and high modulus can be produced by means of the liquid crystal spinning where an inorganic acid is used as a solvent. However, due to their high crystallinity and fibrillation, it is not substantially possible to prepare films. Moreover, it is almost impossible to prepare transparent films.
As a typical example of the high performance heat resistant films which are well known and widely used, there can be mentioned a polyimide film which is commercially available under trade name "Kapton" by E. I. Du Pont de Nemours and Company. The Kapton film has high heat resistance and other required electric porperties, and is used as the passivation layer in the production of insulation materials for the usage at high temperatures, integrated circuits, or flexible circuit boards. The polyimides take an increasing interest in the fields of commerce and industry due to their superior mechanical properties and heat and oxidization resistance, and are now used in the fields of industry such as electrics, electronics, automobils, aeronautic spaces, packaging, etc., in place of the metals and glass.
Polyimides have different physicochemical properties depending on the molecular structures thereof. Particularly, the polyimides drived from aromatic diamines and aromatic dibasic acid anhydrides have the desired properties for use in the high performace utility. The most useful property of aromatic polyimides is the stability to heat and oxidization. For example, polypyromellitimide prepared from pyromellitic acid dianhydride as an aromatic dibasic acid anhydride and m-phenylenediamine or 4,4'-diaminodiphenylether as an aromatic diamine exhibits superior thermal stability (i.e., the weight loss according to the thermogravimetric analysis is 2% or less) at a temperature higher than 500.degree. C. in vacuum or under a nitrogen atmosphere.
In general, polyimides are very stable against hydrolysis and chemicals except for a strong alkali. Especially, polypyromellitimide or polyetherimide with high crystallinity is insoluble in orgarnic solvents, but soluble in strong acids such as sulfuric acid, like the aromatic polyamides mentioned above. Thus, the materials prepared from these aromatic polyimides can desirably be used in the usage where the heat resistance and weather resistance are required. However, it is very difficult to mold them into materials. In fact, it is impossible to mold aromatic polyimides, particularly polypyromillitimide, in a molten condition. Therefore, the two step molding process has been adopted which comprises molding a solution of unstable poly(amidic acid) precusor dissolved in a solvent into a film; and forming imide rings on the film by treating with heat or chemicals. The above post-treatment imidation results in those problems such as complexity in procedures, increasement in manufacturing costs, etc. As an approach to solve these problems, there have been developed the modified polyimide or polyamide films, such as addition polymerization type polyimide films. However, these modified films suffer from poor mechanical properties.
It has been known that the solubility of polyamides and copolyamides having the stiff linear molecular chains extended from both ends of the aromatic ring toward its parallel direction can be increased by rendering the molecular chains flexible at both ends of the aromatic ring. To this end, meta-linkage ring units or rotatable bonds are introduced into a position between the aromatic rings, or pendant units in a suitable size are introduced along the molecular chains. However, in general, introduction of the modified units mentioned above into the copolyamides results in the degradation of physicochemical properties. It is possible to prepare special copolyamides which are not accompanied by the degradation of physicochemical properties. However, the copolyamides are significantly expensive in cost, resulting in economical disadvantages.